Wouldn't you be better to fly there and get a hire car from the nearest airport or town. Or fly into Munich and start your trip from there. I hired a 535d in Munich once - fortunately they don't check the tyre tread depth before and after

You will struggle to find a hire company that will let you take the car out of the UK.

I was looking into something similar, I have always wanted to do a Highlands trip and I am finally going to do it in June next year for four days. I started looking around for sports car hire myself and can't find anything decent for under £1k, plus a £2.5K deposit. I think now I am going to get the boat across with my own car and spend money on some really nice hotels which will keep my wife happy as a road trip is not really her sort of thing.